Human hair naturally grows in a number of shapes. For example, some persons have naturally straight hair, others have naturally curly hair. For fashion, appearance, manageability, or other reasons, persons often desire to change the shape of their hair from curly to various degrees of straight, or vice versa.
The shape of human hair depends on the chemical composition and molecular arrangement of its keratin protein, primarily as it appears in the cells of the cortex. Keratin protein in these cortical cells is present in two different forms: as alpha helices stabilized by intramolecular hydrogen bonds to form stiff rods known as intermediate filaments, and as an amorphous mass in which the intermediate filaments are embedded. Importantly, disulfide bonds serve both to make this amorphous matrix rigid and to connect it to the intermediate filaments. The overall configuration of the hair fiber, curly or otherwise, depends on these structures and the cross-sectional shape of the follicle from which the fiber emerges. Thus, in hair, keratin is a structural protein that is characteristically crosslinked by disulfide bonds which are greatly responsible for its strength and configuration.
During the process known as permanent waving, hair is reacted with a reducing agent such as ammonium thioglycolate thereby breaking a significant number of its disulfide bonds and converting them into the corresponding thiols. Then the hair is forced into a revised configuration by wrapping it around a cylindrical mandrel (curling or waving rod) with some degree of tension, and the thiol groups are reacted with an oxidizing agent such as hydrogen peroxide to form new disulfide bonds which stabilize the revised configuration. In the absence of further chemical treatment, when removed from the mandrel, the hair retains its new, spring-like, curled shape. Correspondingly, this process is known as permanent waving. Although termed permanent, in the presence of water, and/or oxygen, and/or trace minerals and/or sunlight, hair treated by this process may slowly revert to its natural shape through these same reactions, albeit at a greatly decreased rate. Conversely, curly hair can be straightened by this same chemical process by maintaining the hair in a straightened configuration while the disulfide bonds are reformed through oxidation. Similar to hair curled by this process, thus straightened hair tends to revert to its earlier configuration through reverse reactions (disulfide cleavage and re-formation) occurring slowly over a period of time.
The configuration of hair that has been straightened or curled can, however, be stabilized to some degree by subjecting it to temperatures that are adequately high to cause its proteins to crosslink through its carboxyl, hydroxyl and amino groups. The resultant covalent crosslinks are more resistant to cleavage and therefore, more stable than the more chemically reactive disulfide bonds. Relatively high temperatures (150° C. and above) are required to create these crosslinks since most such reactions require dehydration. Given that a significant portion of the rods upon which hair is wound during permanent waving are in close proximity to the scalp it would be dangerous to apply such high temperatures to stabilize the resultant curls. In contrast, this can be achieved during straightening by stretching hair in a direction away from the scalp between heated flat plates in appliances known by such names as flat irons and hair straighteners.
The bulk density and curliness of an individual's hair ranges from thin and straight to dense and curly. The first significant attempts at straightening very dense and curly hair used “hot combs.” During this process the curly hair was softened by the considerable heat of the hot comb and, in effect, stretched to straightness. The high temperature employed caused some degree of protein crosslinking but also rendered the hair brittle and excessively damaged. Often greases, and/or waxes, and/or oils (e.g., lanolin, petrolatum, beeswax, mineral oil) were applied to lubricate hair during the process and to reduce damage during stretching as well as to provide a superficial degree of remedial smoothness and shine.
An improvement over the hot comb procedure was provided by the “lye” based systems containing sodium hydroxide as the active chemical. The more modern versions of these contain 2.0-2.5% (unless otherwise noted, all percentages cited herein reflect weight by weight (w/w) measures) sodium hydroxide in the form of a cream emulsion. The high concentration of alkali in such formulations swells and softens hair to a much greater degree than did the hot comb procedures. Consequently, it can be straightened by relatively gentle stretching at room temperature resulting in improved results and less damage to the hair as compared with the earlier hot comb procedure. Proper post-treatment neutralization of hair is important to recovery of its mechanical properties although most commercial acidic shampoos employed for this purpose are not adequately effective. While lye systems are effective in straightening hair, the alkali is irritating to the scalp and inflicts significant damage to hair, albeit without the level of embrittlement caused by heating appliances.
The straightening of very curly hair and its stabilization is known to be partly due to the formation of lanthionine cross-links which are more resistant to cleavage than disulfide bonds. Desire to minimize the hazards, as well as the negative consumer opinion associated with the use of lye, led to the development of what has been termed the “no lye” systems based on calcium hydroxide and guanidine carbonate. This combination, which must be kept apart prior to use, forms guanidine hydroxide and works by the same mechanism as does sodium hydroxide. Because this combination generates alkali in situ, its main advantage as compared with sodium hydroxide based systems is that the concentration of alkali in the vicinity of the scalp and hair is much lower at any given time and thus less irritating and damaging, respectively.
Apart from individuals with very curly hair, individuals with a lesser degree of curl often desire to straighten their hair either driven by fashion or the desire for improved manageability. Given its lesser degree of curliness, such hair does not require the severe processes described above. The numerous products commercialized for this purpose tend to fall into one of the following two categories: Products that employ a reducing agent such as salts of thioglycolic acid or sodium bisulfite to reduce (cleave) the disulfide bonds, and then oxidize the hair in a new, straightened configuration (as in permanent wave technology); or products that straighten and cross link hair with the use of highly heated (190-230° C.) appliances (e.g., flat iron and hair straightener) with or without the use of cross linking chemicals (most notably formaldehyde and glutaraldehyde). While the latter category provides significantly longer lasting results it requires a far more tedious, lengthy and expensive process (which, typically, must be performed only by a professional); inflicts greater damage to hair; and the agents employed for crosslinking (notably formaldehyde) are noxious and raise toxicological concerns to the extent that they have been banned in a number of countries.